I run two kvm guests on my host, with the same config. One of them is a ubuntu 17.10, the other is a debian 9.2. Both are running on linux kernel 4.13 (debian is installed from the backports repo) On the host side I see, Ubuntu's qemu process consumes nearly no cpu, while debian's uses 25%. On virt-manager's screen on the ubuntu's performance page I see 1-2% cpu usage, but 13% on debian's while they are in idle state.

If I logon to guest and run "top", then I see the system is in idle state. Have you got any idea, why consumes debian's qemu process so much cpu on the host? What can it do while the guest do nothing?

  • What hardware is the host? – Michael Hampton Dec 1 '17 at 19:56
  • An Asus N3150I-C mainboard, 8G RAM, 120GB Kingston SSD. – user446594 Dec 1 '17 at 21:33

It can be to a wrong timer selection and/or a too high polling frequency. Try the following:

  • disable any guest USB controller and device
  • boot Debian with nohz=true
  • in the VM definition, try changing the machine type to something different (for example, pc-i440fx-2.1)
  • Thanks, but it should be a configuration issue, but I don't know, how is it possible :( Some minutes ago I'v shot down both vms and booted the same grml live ISO on them. The vm which normally runs the debian was consuming high cpu in this case too... So it must be my fault, not kvm or debian... :( – user446594 Dec 1 '17 at 20:15
  • Because I had no other idea, I've deleted debian's kvm config, and recreated with virt-manager. Now it is running without any problems, with minimal cpu usage... I have a backup of the original .xml, but there is no significant difference between them, except, the new config has many USB controllers, while the old has only one... – user446594 Dec 1 '17 at 21:26
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    The edit user446594 made probably changed the machine type. E.g., changing from pc-1.1 (ancient) to pc-i440fx-2.1 brought a VM here down from ≈25% CPU to ≈2%. Suggest mentioning that in your answer. – derobert Jun 21 '18 at 21:05
  • @derobert good idea, I've just updated my answer. – shodanshok Jun 21 '18 at 23:13
  • the one I changed it to is newer, not older. But I suppose you could need older, depending on the guest. – derobert Jun 21 '18 at 23:15

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